A survey of histamine levels in plasm, urine and blood in patients with various disorders indicates that the appearance of histamine in plasma is found only in patients with severe mastocytosis, chronic myelogenous leukemia, anaphylactic reactions and during severe urticarial reactions in certain forms of physically induced urticarias. All these patients had symptoms attributable to histamine release, e.g. increased gastric secretion, hypotension. Elevated blood histamine was observed in all cases of chronic myelogenous leuemia and polycythemia vera. Urine histamine was also elevated in these disorders, even in the absence of histamine-like symptoms. Urine histamine measurements were found to be especially useful in distinguishing patients with cutaneous mastocytosis from those with systemic mastocytosis. BIBLIOGRAPHIC REFERENCES: Metzger, W.J., Kaplan, A.P., Beaven, M.A., Irons, J.S.and Patterson, R.: Hereditary vibratory angioedema: Confirmation of histamine release in a type of physical hypersensitivity. J. Allergy Clin. Immunol. 57: 605-608, 1976. Wilcox, G. and Beaven, M.A,: A sensitive and specific tritium assay for dopamine-beta-hydroxylase (D Beta H) in serum. Anal. Biochem. 75: 484-497, 1976.